Improved trait coherence checking

[tritao]

Description

This PR refines trait coherence checking to guarantee that there is only one valid implementation for each trait. It introduces improved orphan rules and more rigorous overlap checks to prevent conflicting trait implementations. With these changes, the system now more reliably enforces coherence, ensuring that traits are implemented uniquely, and
reducing potential ambiguities.

This approach mirrors Rust’s design by enforcing two distinct safety and coherence checks on trait implementations:

1. Orphan Rules Check
The orphan rules require that for any trait implementation, either the trait or the type must be defined within the current package. This restriction prevents external packages from implementing foreign traits for foreign types, which could otherwise lead to conflicting implementations and ambiguities across different parts of a codebase. Essentially, it helps maintain clear ownership and boundaries of trait implementations.

2. Overlap Impl Check
The overlap impl check ensures that no two trait implementations can apply to the same type in an ambiguous manner. If two implementations could potentially match the same type, it would be unclear which one should be used, leading to coherence issues. By enforcing that implementations do not overlap, the system guarantees that trait resolution is unambiguous and predictable.

Together, these checks promote a robust and maintainable system by ensuring that trait implementations are both locally controlled (orphan rules) and non-conflicting (overlap check).

The test suite has been updated to comply with the new rules. However, there is one current limitation regarding arrays. For arrays, the coherence checks have been relaxed to avoid the need for numerous concrete implementations in the standard library for traits like Eq and PartialEq. This decision was made because support for const generics is expected soon, which will allow these traits to be implemented more cleanly.

Closes issue #5892.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6844 will degrade performances by 50.73%

_{Comparing tritao:trait-coherence (f770057) with master (fccc8a9)}

Summary

❌ 2 regressions
✅ 20 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	compile	4.7 s	9.6 s	-50.73%
❌	hover	3.4 ms	4 ms	-16.54%